1. Field of the Invention
The present invention relates generally to a rotation control apparatus suitable for use with a recording medium such as a magnetic or optical type of disk and, more particularly, to a rotation control apparatus for providing servo control over the rotation of such a recording medium which allows signals to be read thereto or written therefrom.
2. Description of the Related Art
For example, in a typical electronic still camera, rotation control is provided as shown in FIG. 3. As illustrated, a magnetic disk 10 is caused to rotate by a motor 12, and a PG coil 16 is adapted to detect the presence or absence of a small magnet 14 attached to one side of the magnetic disk 10. If the PG coil 16 detects the presence of the small magnet 14, the PG coil 16 supplies a PG pulse as a feedback signal to one input of a spindle servo-circuit 18, at which time a reference signal S.sub.3 is supplied to another input of the spindle servo-circuit 18. The spindle servo-circuit 18 controls the operation of the motor 12 so that the phase difference between the PG pulse and the reference signal S.sub.3 may be kept constant.
In this state, if a recording mode is selected by operating a switch (not shown), an analog switch 20 is switched to a REC position, and a vertical-synchronizing-signal separating circuit 22 separates an external vertical synchronizing signal S.sub.1 from a composite color video signal. The external vertical synchronizing signal S.sub.1 is supplied to the reset terminal VR of a synchronizing-signal generator 24 and thus a vertical synchronizing signal S'.sub.1 whose phase has been reset by the external vertical synchronizing signal S.sub.1 is supplied as the aforesaid reference signal S.sub.3 from the output terminal VD of the synchronizing-signal generator 24 to the spindle servo-circuit 18 through the analog switch 20 which is switched to the REC position.
On the other hand, if a reproduction mode is selected by operating the switch (not shown), the analog switch 20 is switched to a PB position, and a subcarrier of 3.58 MHz is supplied from the synchronizing-signal generator 24 to a frequency division circuit 26. The frequency division circuit 26 effects the frequency division of this 3.58-MHz subcarrier and outputs an internal vertical synchronizing signal S.sub.2. This signal S.sub.2 is also supplied as the aforesaid reference signal S.sub.3 to the spindle servo-circuit 18 through the analog switch 20 which is switched to the PB position.
In this fashion, the magnetic disk 10 is rotated on the basis of the external vertical synchronizing signal S.sub.1 during recording, while, during reproduction, the magnetic disk 10 is rotated on the basis of the internal vertical synchronizing signal S.sub.2.
In such a related art electronic still camera, however, switching between the production and recording modes involves the following problems.
If the reproduction mode is switched to the recording mode, the period of the reference signal S.sub.3 abruptly varies as shown in FIGS. 4(a) to 4(c), since the vertical synchronizing signals S'.sub.1 and S.sub.2 differ in phase from each other. It is impossible, therefore, to initiate recording until servo-control reaches its stable state.
On the other hand, if the recording mode is switched to the reproduction mode, the period of the reference signal S.sub.3 likewise varies abruptly and it is impossible to perform proper reproduction until servo-control reaches the stable state.